Voice/audio will never be better than text (email) because RAM is always better than SAM (sequential access memory). When you are listening to some audio, you are processing information sequentially and you have to listen to the whole thing to get it. When you have the entire text in front of you, you can jump around as needed, to speed up processing. Not just skipping ahead to get past overly verbose explanations, but also going back a sentence or two for a second read in case you are not quite getting the point. Try that with a voicemail: "What did he mean by "that other time"?! ah, right, I think he was going on about it earlier. I guess I'm going to have to listen to the whole thing one more time. Dammit, I spaced out again during that long tangent, what was the point he was trying to make after all? I guess I'm going to have to listen to the whole thing the third time."
Also, when you are the one doing the reading, you have full control over the speed. You can slow down during complicated parts, giving yourself time to get it, and speed up over trivial stuff. Not so much with voicemail: can't just slow down someone's speech, or speed it up as needed.